The most important thing is that commedia dell’arte was part of an oral tradition that was hundreds and possibly thousands of years old (and still exists today in little shrinking pockets of European culture, in the former Yugoslavia for example) in which poets, singers, storytellers, jongleurs, troubadours and actors used formulas of plot, story structures and grammars, character types, little ‘building bricks of language’ (epithets, adjectival, adverbial and noun phrases) and memorised set speeches and speech styles, which were so deeply rooted in their professional and family inheritance they were really good at it, apparently spontaneously creating spoken text which was complex and beautiful. Homer’s ‘Odyssey’ and ‘Iliad’ (though of course a much earlier part of the oral tradition) were probably produced in exactly this way, which illustrates how ‘rich’ the commedia dell’arte text might have been.
So although it was mainly improvised it was really rich text created from a kind of cyberspace of memory-held language from a culture that relied much less on written text than we do today. Many of the performers were also multi-lingual, highly educated and sophisticated people, but still itinerant in lifestyle and contiguous with the oral and non-literate traditions.
According to Antonio Fava the magnificent Isabella Andreini of The Gelosi was able to freely improvise sophisticated rhyme. There are of course very few commedia dell’arte performers left who have any kind of ‘blood line’ to this tradition – Antonio Fava is one (having inherited his father’s Pulcinella show), Franca Rama (Dario Fo’s wife) is another, the Carrara family of Vicenza, another well known link to the tradition, but most of the rest of us are all ‘fast tracked’ in the tradition and simply do not have all this rich humus of material to improvise with in the way the Gelosi, the Intenti, and the Dediosi troupes, and great actors, like Isabella Andreini, Zan Ganassa and Tristano Martinelli, did.